(1) Field of the Invention
The present invention relates to a method of generating tightly intermeshing, self-cleaning, co-rotating screw profiles, a computer program product for carrying out the method in a computer system, novel screw profiles generated by the method and the use of the screw profiles obtained in screw and transition elements.
(2) Description of Related Art
Those skilled in the art are familiar with co-rotating twin- and multiscrew extruders from the patent and technical literature. The following publication can be mentioned as an example: K. Kohlgrüber: “Der gleichläufige Doppelschneckenextruder”, (“The co-rotating twin-screw extruder”), Publishers: Hanser Verlag, 2007. This publication explains in detail the design, function and operation of twin- and multiscrew extruders. An entire chapter (pages 227-248) is devoted to the subject of screw elements and their mode of action. A detailed explanation is provided of the design and the function of conveying, kneading and mixing elements. In order to provide a transition between screw elements with different numbers of flights (or screw channels), washers are frequently used as spacers. In special cases, so-called transition elements are used which provide a continuous transition between two screw profiles with different numbers of flights, a self-cleaning pair of screw profiles being present at each transition point. When mentioned and described in the following, the terms screw elements and screw profiles are also understood to include transition elements and their profiles. The profiles of the transition elements are also referred to as transition profiles.
As is well-known to those skilled in the art and as is explained, for example, on pages 96 to 98 of Kohlgrüber, the known self-cleaning Erdmenger screw profile is clearly defined by the following three parameters: the number of flights z the outer screw radius ra and the centre distance a. The number of flights z is an integer greater than or equal to 1. An additional important parameter of screw profiles is their inner radius ri. Another important parameter of screw profiles is their flight depth h. (For the sake of clarity, in the description all abbreviations, symbols and indices used are written in italics and in the figures all abbreviations, symbols and indices used are written in normal script.)
As is well-known to those skilled in the art and as is explained, for example, on pages 96 to 98 of Kohlgrüber, the known self-cleaning Erdmenger screw profile consists of arcs of circles. The size of an arc is defined by its central angle and its radius. In the following, the “central angle of an arc” is abbreviated to the “angle of an arc”. The position of an arc is defined by the position of its central point and that of its starting or end point. The position of the starting point and the end point of an arc is, however, not predefined, since an arc can begin or end either in a clockwise or an anti-clockwise direction. The starting and end points are therefore interchangeable.
The methods so far known for producing tightly intermeshing, self-cleaning, co-rotating screw profiles have the disadvantage that they are mathematically complicated and always associated with specific screw profiles, such as, for example, the Erdmenger screw profile. Known processes for producing Erdmenger screw profiles are described for example in Kohlgrüber or in the publication by Booy: “Geometry of fully wiped twin-screw equipment”, Polymer Engineering and Science 18 (1978) 12, pages 973-984. In the aforementioned publications screw profiles are generated by making use of the kinematic peculiarity that the same sense rotation (=co-rotation) of two screws about their stationary axes is kinematically identical to the “movement without rotation” of one screw about another, in this case stationary, screw. This phenomenon can be used for the stepwise generation of screw profiles. The first screw (the “generated” screw) remains stationary using such a method and the second screw (the “generating” screw) is moved translationally around the first screw on an arc. It is then possible to predefine part of the profile in the second screw and examine which profile is thereby generated in the first screw. The generated screw is “carved” by the generating screw. Kohlgrüber does not, however, describe how the predefined part of the second screw is actually to be generated. Booy describes a possible method of generating the starting profile section from which the remaining profile is generated. This method is, however, mathematically very complicated and above all not universally applicable, i.e. it is not possible to generate any desired type of profile for screw and transition elements using this method.
Based on the prior art, the problem therefore arises of providing a method for generating screw profiles in which screw profiles can be generated without any predefined existing profiles and/or profile sections. The problem also arises of providing a method of generating any desired profiles for tightly intermeshing screw and transition elements. The problem also arises of providing a simple method of generating profiles for tightly intermeshing screw and transition elements. This method must be capable of being carried out by the mere use of a pair of compasses and an angle ruler without the need for complicated computations.